The present invention relates to an improvement in regulated power supplies and, more particularly, to an improvement in power supplies for developing a highly accurate voltage magnitude for application to a cyclical load such as a flash lamp.
Various types of power supplies are used in cyclical load applications. Generally, all these power supplies utilized some means of charging an energy storage capacitor to a predetermined voltage magnitude. The energy in the capacitor is then discharged through the load when desired. In flash lamp applications, the energy discharge is converted to light and heat by discharge through the "arc" lamp.
A typical power supply for a flash lamp uses "on-off" primary control and a high leakage reactance power transformer. In such a system, a regulating circuit monitors the voltage developed on the output capacitor and interrupts the primary winding of the transformer when the capacitor voltage reaches the predetermined magnitude. Interruption of power to the primary winding is effected by removing a gate signal from a triac which is connected between the primary winding and an AC source. Capacitor charging is thereby terminated and the capacitor remains charged until called upon to discharge through the flash lamp. This circuit performs well in applications not requiring precise regulation of the energy in the capacitor. Because of the method of control, energy is delivered to the capacitor in half-cycle increments of the applied AC voltage. Since the triac switch is not force commutated when its gate signal is removed, the charging process cannot be stopped until the primary winding current passes through zero. Under worst case conditions, energy could be delivered to the capacitor for nearly a full half-cycle of the applied AC voltage after the triac gate signal has been removed. This excess energy would then result in higher light intensity being developed by the flash lamp. When the flash lamp is part of a photocopier, the increased light intensity will result in overexposure of the photocopy. Adjustments to compensate for such overexposure are impractical since the time at which the gating signal is removed with respect to the phase angle of the AC source voltage is unpredictable.